1. Field of the Invention
The present invention relates to a photo mask used in a photolithographic process for fabricating semiconductor devices, in particular a photo mask wherein light blocking regions of a mask pattern with a large pitch can be formed with a small opening to improve the depth of focus of the mask pattern.
2. Discussion of Background
As semiconductor devices are required to operate faster and to be formed in high integration, there has arisen a demand for provision of micro geometry to patterns. The micro geometry is required to be provided on a plane with steep steps in high relief with good precision, which requires not only good resolution but also great depth of focus. In general, a photo mask which is used in a photolithographic process has large and small patterns mingled on a single plane. The patterns vary in size from fine geometry of patterns with a small pitch such as wiring to patterns with a large pitch such as peripheral circuits. With regard to such fine geometry of patterns, there have been proposed various high resolution techniques which can improve depth of focus and resolution as stated later. However, such techniques can't give a desired effect to patterns with a large pitch, creating a problem in that it is impossible to obtain depth of focus to cope with steep steps in high relief.
Now, a photolithographic process using a conventional photo mask will be explained.
First, a transferring step for a mask pattern by a projection exposure system using a conventional circular light source method will be described.
In FIG. 15, there is shown a schematic view of the structure of a reduction-type projection exposure system according to a conventional circular light source method, which is used in a photolithographic process. Now, a step to transfer a mask pattern onto a wafer by the reduction-type projection exposure system will be explained.
Referring to FIG. 15, light which has been emitted from a circular light source 6 (the optical path of the light is indicated by solid lines) irradiates the entire plane of a photo mask 1 at an angle perpendicular thereto after the light has been condensed by a condenser lens 7. The light which has irradiated the photo mask 1 goes out in a direction depending on a pitch of a mask pattern 3 formed on the photo mask 1 after the light has been diffracted by the mask pattern. The outgoing diffracted light is converged by a projection lens 7'. The diffracted light is reduced by the projection lens 7' after the light has passed through a pupil plane 8 in the projection lens. The reduced light forms an optical image of the mask pattern 3 at a best focused position on the wafer 4. The wafer 4 has had e.g. a resist coated thereon. A patterned resist 5 is obtained so as to correspond to the mask pattern by subjecting the resist on the wafer to an ordinary photolithographic process.
Next, the relationship between the light diffracted by the photo mask and the pitch in the mask pattern will be described in detail.
In FIGS. 16 and 17, there are shown enlarged plan views of mask patterns which are arranged on the photo mask. The respective views show a pattern which has a large pitch in an x direction and a pattern which has a small pitch near to a resolution limit in the x direction. In FIGS. 18 and 19, there are shown cross-sectional views taken along the line III--III of FIG. 16 and taken along the line IV--IV of FIG. 17. In these Figures, reference numeral 1 designates the photo mask, reference numeral 2 designates a transparent substrate, reference numeral 3 designates light blocking regions which are formed by a thin film made of metal such as chromium, and reference numeral 9 designates an opening which is arranged between the light blocking regions 3.
In the mask pattern shown in FIG. 16, a plurality of rectangular light blocking regions 3a are periodically arranged at a large pitch P.sub.xL in the x direction and at a pitch P.sub.yL in a y direction. In the mask pattern shown in FIG. 17, a plurality of line arrays of light blocking regions 3b are periodically arranged in a line and space pattern with a small pitch P.sub.xs in the x direction.
In FIGS. 20 and 21, there are shown schematic diagrams wherein the optical paths of the light diffracted by the mask patterns of the photo mask shown in FIGS. 16 and 17 are indicated. In FIGS. 20 and 21, reference numeral 1 designates the photo mask, reference numeral 3 designates the mask patterns, reference numeral 4 designates the wafer, reference numeral 7' designates the projection lens, and reference numeral 8 designates the pupil plane. Reference numeral 50 designates zero order diffraction light, reference numerals 51 and 52 designates -first order diffraction light and +first order diffraction light, respectively, and reference numerals 53 and 54 designates -second order diffraction light and +second order diffraction light, respectively.
The light (irradiated light) which is incident on the photo mask is diffracted by the mask patterns, and an outgoing angle of each diffraction light with respect to the optical axis is determined by the pitches of the mask patterns and the order of the diffraction light. In general, when the outgoing angle of a first order diffraction light with respect to the optical axis is defined as .theta., the wavelength of the light is defined as .lambda. and the pitch of the mask patterns is defined as P, there is a relationship of Sin.theta.=.lambda./P. The outgoing angle becomes greater as the order becomes higher.
Referring to FIG. 20, when the pattern pitch is large, the outgoing angle of the -first order diffraction light 51a and the outgoing angle .theta. of the +first order diffraction light 52a symmetrical thereto with respect to the optical axis become smaller, and the outgoing angles of higher order diffraction light such as the -second order diffraction light 53a and the +second order diffraction light 54a symmetrical thereto with respect to the optical axis become also smaller, so that higher order diffraction light can pass through the pupil plane 8 of the projection lens 7'. Each diffraction light which has passed through the pupil plane 8 causes interference to form an optical image at a best focused position on the wafer 4.
Referring now to FIG. 21, when the pattern-pitch is small and near to a resolution limit, the outgoing angle of each diffraction light becomes larger, causing higher order diffraction light such as the second order diffraction light 53a and 54a to deviate outside of the pupil plane 8, and allowing only the zero order diffraction light 50 and the first order diffraction light 51 and 52 to pass through the pupil plane 8.
As explained, the outgoing angle of each diffraction light varies depending on the size of a pattern pitch. As the pitch becomes larger, the optical image can be formed by multiple order diffraction light.
In FIGS. 22(a) and (b), there are shown schematic views wherein the diffraction light shown in FIGS. 20 and 21 is viewed as light source images on the pupil plane. In FIGS. 22(a) and (b), reference numeral 60 designates the light source image by the zero order diffraction light, reference numerals 61 and 62 designates the light source image by the -first order diffraction light and the light source image by the +first order diffraction light, respectively, and reference numerals 63 and 64 designates the light source image by the -second order diffraction light and the light source image by the +second order diffraction light. Reference numeral 8 designates the pupil plane.
As shown in FIGS. 22(a) and (b), the center distance between the light source image 60 by the zero order diffraction light and the light source images 61 and 62 by each first order diffraction light is equal to .lambda./P, and the radius of the pupil plane 8 is equal to the numerical aperture NA of the projection lens. The center of the respective light source images aligns with the optical axis of each diffraction light shown in FIGS. 20 and 21.
Referring to FIG. 22(a), when the pattern pitch is large, the light source images 61-64 by each diffraction light gather towards the center of pupil plane 8 so as to overlap one another as the outgoing angle of each diffraction light becomes smaller as explained with respect to FIG. 20. In the case of FIG. 22(b), since the pattern pitch is small, the outgoing angle of each diffraction light becomes larger, causing the respective light source images to be apart from the center of the pupil plane 8, and thereby completely deviating the light source images 63 and 64 by each second order diffraction light from the pupil plane 8.
Now, the size of a pattern pitch will be explained.
FIGS. 23(a)-(c) are schematic diagrams showing how a light source image by each diffraction light is obtained depending on a change in a pattern pitch. In FIG. 23(a), there are shown the light source images which are obtained when the pattern pitch is set just to he resolution limit, wherein the center of the light source images 61 and 62 by each first order diffraction light is located on an outer circumference of the pupil plane 8 and the light source images 63 and 64 by each second order diffraction light are completely deviated from the pupil plane 8. If the pattern pitch becomes smaller, each first order diffraction light 61 and 62 is also gradually deviated from the pupil plane 8, making formation of an optical image difficult. This means that at least interference between the zero order diffraction light and either one of the .+-.first order diffraction light is required to form an optical image with contrast in brightness. When the pattern pitch which is set to the resolution limit is defined as P, the numerical aperture of a projection exposure system is defined as NA and coherency is defined as .sigma., the following relationship is found: EQU P=.lambda./NA (1)
In FIG. 23(b), it is shown how the light source images are obtained when the pattern pitch becomes larger than that in FIG. 23(a) and be near to the resolution limit, wherein the light source images by each first order diffraction light gather near to the center of the pupil plane 8 while the light source images 63 and 64 by each second order diffraction light are still deviated from the pupil plane 8. In this case, the light source images 63 and 64 by each second diffraction light are circumscribed with the outer circumference of the pupil plane 8, which shows the minimum pitch at which each second diffraction light is completely deviated from the pupil plane. When the pitch in this case is defined as P, the following relationship is found: EQU P=2.lambda./NA(1+.sigma.) (2)
In FIG. 23(c), it is shown that the pattern pitch becomes larger than that in FIG. 23(b), and the light source images 63 and 64 by each second order diffraction light enter the pupil plane 8 to make a little contribution to formation of the optical image. When the pattern pitch in this case is defined as P, the following relationship is found: EQU P&gt;2.lambda./NA(1+.sigma.) (3)
Accordingly, a small pattern pitch which is near to the resolution limit can be defined as a pitch satisfying the following formula based on the formulas (1)-(3), i.e. a pitch which prevents at least the light source images by each second diffraction light from entering the pupil plane and which can form an optical image: EQU .lambda./NA.ltoreq.P&lt;2.lambda./NA(1+.sigma.) (4)
A large pitch can be defined as a pitch satisfying the formula (3), i.e. a pitch which allows the second order diffraction light or more to enter the pupil plane even a little to contribute to formation of an optical image.
Now, the relationship between diffraction light and depth of focus will be described.
In general, the depth of focus shows what contrast in brightness a reproduced optical image has at defocused positions under and above a best focused position on a wafer as a reference.
For example, when the pattern pitch is small and only the zero order diffraction light 50 and the .+-.first order diffraction light 51 and 52 pass through the pupil plane 8 as shown in FIG. 21, such three kinds of diffraction light travel along different optical paths as zero order diffraction light 50b, and first order diffraction light 51b and 52b, and form an optical image on the best focus position on the wafer 4 by interference among three kinds of diffraction light (it is called 3 beams interference). At positions which are defocused upward and downward from the best focus position, the contrast in the optical image is gradually lowered depending on defocusing since the zero order diffraction light 50b and each first order diffraction light 51b and 52b are out of phase (the -first order diffraction light 51b and the +first order diffraction light 52b are in phase because they are symmetrical to each other with respect to the optical axis).
When the pattern pitch is large and the -second diffraction light 53 and the +second diffraction light 54 together with the zero order diffraction light 50 and the .+-.first order diffraction light 51 and 52 pass through the pupil plane 8 as shown in FIG. 20, the second diffraction light having a different optical path contributes to formation of the optical image to further lower the contrast in the optical image. When the pattern pitch becomes much larger, third order diffraction light, fourth order diffraction light or more are added to furthermore lower the contrast though not shown.
As explained, in a pattern with a large pitch, the depth of focus is lowered. Simulation results of the depth of focus obtained by the shown mask patterns will be indicated.
In FIGS. 24 and 25, there are shown the results which were obtained by simulating a light intensity distribution of an optical image formed by the mask pattern with a large pitch shown in FIG. 16 on best focusing and on defocusing.
The mask pattern which was used in this simulation was the same as the mask pattern shown in FIG. 16, wherein the pitch P.sub.xL was 1.5 .mu.m and the opening between the light blocking regions had a spacing D.sub.x of 0.3 .mu.m. The optical conditions were that the wavelength of exposure light was 248 nm(KrF excimer laser), the numerical aperture NA was 0.55 and the coherency .sigma. was 0.8. When the best focused position in FIG. 24 was zero .mu.m, the defocused value in FIG. 25 was 0.4 .mu.m. The ordinates in both Figures represent standardized light intensity wherein 1 represents the maximum intensity and degrees of intensity are classified with tones.
When the light intensity distribution in the x direction is viewed referring to FIGS. 24 and 16, there is a gentle gradient from the weakest light intensity portion corresponding to a central portion of the light blocking region 3a having of an intensity of 0-0.1 to the strongest light intensity portion corresponding to an opening around the light blocking region. A spacing d.sub.1 in the x direction between the central portion regions having the weakest intensity is about 0.5 .mu.m. Referring now to FIGS. 25 and 16, the light intensity distribution on defocusing was characterized in that the intensity gradually become stronger from a central portion to a peripheral portion of a light blocking region as on best focusing, and that a spacing t.sub.2 in the x direction between the central portion regions having the weakest intensity of 0-0.1 was about 0.8 .mu.m, which showed that the spacing was extended in comparison with the spacing on best focusing. This means that the gradient of the intensity distribution in the x direction become gentler to make the contrast lower.
As explained, the depth of focus is extremely lowered in a mask pattern with a large pitch because an optical image is formed by a plurality of kinds of diffraction light having different optical paths (it is called multiple flux interference).
With respect to a mask pattern with a smaller pitch, there have been proposed techniques which further improve the depth of focus.
For example, "Illumination Modification of an Optical Aligner" KTI Microelectronics Seminar, pp. 217-230 (1989) has proposed a modified illumination method which is illustrated in FIG. 26 showing a schematic view of light paths of light diffracted by a photo mask. The modified illumination method is a technique which controls diffraction light caused by a pattern having a small pitch near to resolution limit to improve the depth of focus.
In this Figure, the photo mask 1 is the same as the conventional photo mask shown in FIG. 17, and has a line and space pattern 3b arranged thereon at a pitch P.sub.xs near to resolution limit. The other reference numerals indicate parts identical or corresponding to those indicated by the reference numerals in FIG. 21.
Referring to FIG. 26, according to the modified illumination method, the zero order diffraction light 50 from the photo mask 1 and only one of the .+-.first order diffraction light 51 or 52 (e.g. first order diffraction light 52) enters the pupil plane 8 of the projection lens 7, and the incident angle .alpha. of incident light is selected so as to symmetrically pass the zero order diffraction light and the first order diffraction light through the pupil plane 8 in the 3 beams interference of a pattern having a small pitch P.sub.xs near to the resolution limit (formation of an optical image by interference among the zero order diffraction light and the .+-. first order diffraction light). By this method, the number of interfering diffraction light is decreased to have so-called two luminous flux interference, and two kinds of diffraction light which form an optical image have symmetrical light paths, allowing the depth of focus to be improved without lowering the contrast in brightness of the optical image even at a defocused position.
For example, "Improving Resolution in Photolithography with a Phase Shifting Mask", IEEE Trans. Electron Devices, ED-29, pp. 1828-1836 (1982) has proposed a Attenuated phase shifting technique which is illustrated in FIGS. 27(a), (b) and (c) showing a cross-section view of a photo mask and schematic views of the intensity of the light transmitted through the mask.
In the Attenuated phase shifting technique, diffraction light caused by a pattern having a small pitch near to resolution limit is controlled to improve resolution and depth of focus as in the modified illumination method. As in the modified illumination method, the photo mask shown in FIG. 27(a) is the same as the conventional photo mask shown in FIG. 16, and the photo mask has a line and space pattern arranged thereon at a small pitch P.sub.xs near to the resolution limit.
In the Attenuated phase shifting technique, either one of openings adjacent to a mask pattern 3b with a small pitch on the photo mask 1 is formed with a thin transparent film 90 (see FIG. 27(a)) to shift the phase of transmission light 56 with respect to transmission light 55 adjacent thereto by 180.degree., thereby cancelling the amplitude of light at an overlapped portion of light amplitude 70 and light amplitude 71 of each transmission light (see FIG. 27(b)) to obtain light intensity 80 with good resolution (see FIG. 27(c)). By the 180.degree. of shift, the zero order diffraction light from an opening and the zero order diffraction light from an adjacent opening are cancelled together to establish the two luminous flux interference due to only the .+-.first order diffraction light having symmetrical optical paths for formation of an optical image, and simultaneously improving depth of focus.
As another conventional measure, there has been a technique in JP-A-736174 wherein a rectangular pattern has openings formed in a stripe-like shape to improve resolution of fine geometry.
The conventional techniques as stated earlier improve the depth of focus and the resolution by controlling diffraction light caused from a mask pattern having a small pitch near to the resolution limit. However, with regard to a mask pattern having a greater pitch beyond the resolution limit, the modified illumination method can't decrease diffraction light because the optical image is formed by multiple order diffraction light, and the phase shifting method is difficult to apply because the presence of a large distance between adjoining openings prevents light from interfering.